The present invention deals with the field of magnetic iron oxides useful in the tape recording industry. It presents a novel cobalt modified acicular gamma ferric oxide and a novel process for its production, both of which are superior to any such products or processes previously known in the field. Many suggestions in the prior art, for example, Gruber et al. (U.S. Pat. No. 3,075,919), relate to the synthesis of cobalt modified acicular .gamma.-Fe.sub.2 O.sub.3 by alleged co-precipitation of a cobalt containing ferrous hydroxide from an alkali and soluble ferrous and cobaltous salt which is then oxidized to cobalt modified ferric hydroxide. The precipitate is washed, dried and converted to cobalt containing .gamma.-Fe.sub.2 O.sub.3 by heating and then reducing to Fe.sub.3 O.sub.4 and subsequently oxidizing to cobalt containing .gamma.-Fe.sub.2 O.sub.3. When the alkali is added to the soluble ferrous and cobaltous salt, coprecipitation does not occur. As the pH increases, the ferrous ions are precipitated and then the cobalt ions. The presence of the cobalt salts tends to destroy the acicularity of the initial seed resulting in a cobalt modified .gamma.-Fe.sub.2 O.sub.3 which is difficult to orient and results in inferior recording properties when incorporated into magnetic tape.
Some prior art proposals refer to precipitation methods which take place at high pH, Toda et al. (U.S. Pat. No. 3,720,618). These methods produce cobalt modified .gamma.-Fe.sub.2 O.sub.3 which itself has a high pH. It will be understood by one skilled in the art that the pH of a solid is determined from pH measurements of an aqueous suspension thereof using a glass electrode. The high pH of the cobalt modified .gamma.-Fe.sub.2 O.sub.3 is a disadvantage in that these particles are then incompatible with the organic formulation which is used to incorporate the particles into a magnetic recording tape. This is especially true for polyurethane type formulations. Another prior art suggestion (Abeck et al., U.S. Pat. No. 3,117,933) employs a pH which is not taken sufficiently high to completely precipitate all the cobalt ions.
There is much concern in the literature (Haller et al., U.S. Pat. No. 3,725,126) over the stability of cobalt modified .gamma.-Fe.sub.2 O.sub.3 particles in that if information is recorded on them and they are heated to elevated temperatures, a certain loss occurs which is greater than for conventional low coercive force .gamma.-Fe.sub.2 O.sub.3 particles. Some attempts have been made to reduce this instability (Haller, et al., U.S. Pat. No. 3,725,126).